NEW YORK (Reuters) - About one million spectators, mostly dressed in green, streamed into New York on Monday for its St. Patrick's Day Parade, even as the city's mayor and beer companies that previously sponsored the event dropped out amid concerns that organizers excluded gay groups.

Parade organizers in New York and Boston, two of the most liberal U.S. cities, have long excluded openly gay marchers, saying that doing so would conflict with the group's Roman Catholic heritage.

Protests over the exclusion came to a head this year, with newly elected mayors Bill de Blasio of New York and Marty Walsh of Boston skipping their cities' parades in protest. On Sunday, brewery Guinness said it would join Heineken in dropping sponsorship of the parade as it faced protests from local gay rights groups.

On the sidelines of the parade in New York, gay rights groups staged a small but fervent protest on Monday urging people to boycott marching or watching.

"Those who stand with the parade are making a clear choice to endorse bigotry," said Emmaia Gelman of the group Irish Queers, which organizes yearly protests of the parade. "When sponsors finally stop paying for religious-right homophobia, the parade can go back to being Irish."

Organizers of the 253-year-old parade said that gay and lesbian marchers could participate, but not carry signs or banners that expressed gay-rights messages.

"Where will this end?" Murdoch mused on Twitter. "Guinness pulls out of religious parade bullied by gay orgs who try to take it over. Hope all Irish boycott the stuff."

As the floats, bands, dancers and Irish-American groups made their way down Fifth Avenue in Manhattan to the sound of bagpipes and loud cheers from the crowd, some onlookers said they had put concerns aside to enjoy the spectacle.

"I have a son who is gay," said Bridget Farrell, who had traveled to New York from her home in Dublin to see the parade. "Everybody has the right to parade. As far as I'm concerned everyone is equal."

Threatened boycotts by gay rights groups in the days leading up to the events prompted sponsors of the New York and Boston parades to drop their financial backing for the event. Diageo, parent company of Guinness, whose dark, creamy stout is a fixture of St. Patrick's Day parties, said in a statement it hoped the New York policy would change by next year.

Heineken and Sam Adams brewer Boston Beer Co cited concerns about exclusionary policies when they dropped their respective support of the New York and Boston events.

Outside New York's St. Patrick's Cathedral, Cardinal Timothy Dolan, the archbishop of New York, watched the parade but did not directly comment on the controversy.

"It's a celebration of New York, this tremendous tapestry and this diversity all rallying around," he said.

Attitudes on homosexuality have changed dramatically in the United States over the past decade, with 17 states and the District of Columbia now allowing same-sex couples to wed.